Commercial applications for vinyl dispersion resins most often involve the formulation and processing of fluid dispersions known as plastisols. Plastisols are generally produced by mixing the dry resin into a suitable liquid plasticizer or plasticizer system in such a way as to break-up loose agglomerates and disperse the primary resin particles thoroughly and uniformly.
A multiplicity of plasticizers is available having solvating action toward vinyl resins so that the choice and proportions thereof may be selected in the light of the rheological properties desired for the particular processing technique to be used in fabrication of the end product. In addition to the various substrate coating methods, other important processing techniques include casting, extrusion and several types of molding operations (e.g. rotational, dip and slush).
Although there is some molecular interaction between the vinyl chloride polymer and generally used, compatible plasticizers even at the rather moderate temperatures involved in preparing plastisols, maximum strength is developed in the applied and/or shaped material by heating same to temperatures sufficient to achieve complete mutual solvation of plasticizer and resin, with effective fusion into an integral, solidified polymeric matrix. Temperatures above 300.degree. F. are generally employed (e.g. 325.degree.-400.degree. F.) in order to provide good cures in reasonable times.
Most finished plastisol materials are rather soft and fairly flexible, as these properties can be readily obtained by suitable formulation choices, e.g. the grade of resin used and the type and amount of plasticizer combined therewith. Although the softness and flexibility of vinyl plastisol products is impressive considering the inherent stiffness and hardness of traditional rigid vinyl chloride polymers, their elasticity, resiliency and tactile quality are generally marginal, if not actually deficient, for certain potential end-use applications. A good example of such an end-use is the dip forming of hollow, thin-walled items such as disposable gloves for the medical profession. Such gloves prepared from previous vinyl chloride plastisols are notably deficient in elasticity, pliability and aesthetic tactile qualities when compared to gloves made from natural rubber latex.
One method for obtaining modified vinyl dispersion resins providing improved resilience and elasticity is disclosed and claimed in U.S. application Ser. No. 208,921, filed June 20, 1988 in the name of Ladd J. Horvath and Bela K. Mikofalvy. The improved resins of said application are made by spray drying a latex mixture formed by blending, into an emulsion polymerized vinyl chloride latex, a minor proportion of an acrylic rubber latex derived from the following monomers:
(a) about 80 to about 98 weight parts of alkyl acrylates averaging at least 2 carbon atoms in the alkyl group; PA0 (b) about 1 to about 10 weight parts of unsaturated dicarboxylic acids which contain between 4 and about 8 carbon atoms; and PA0 (c) optionally up to 19 weight parts of compatible, additional unsaturated monomers. PA0 (a) about 80 to about 98 weight parts of alkyl acrylates averaging at least 2 carbon atoms in the alkyl group; PA0 (b) about 1 to about 10 weight parts of unsaturated dicarboxylic acids which contain between 4 and about 8 carbon atoms; and PA0 (c) optionally up to 19 weight parts of compatible additional unsaturated monomers.
However, the improved resins so prepared exhibit inadequate stability in regard to the rheological properties of plastisol formulations made therefrom. Thus, comparing two freshly mixed plastisols (composed of equal parts by weight of resin and plasticizer), one made with resin freshly prepared in accordance with U.S. Ser. No. 208,921 and the other made with a retained portion of the same resin after a week or more, the viscosity of the latter is typically several fold that of the former (e.g. about 2 to 6 times as much depending largely upon the actual ambient weather conditions.